1. Field of the Invention
The invention relates to a power supply system, and more specifically, it relates to a power supply system wherein an output current from a generator section is maintained as constant to thereby secure stable operation of the generator section, and load current fluctuations are selectively compensated for by using an output from a power storage section, thereby enabling the outputs to function in cooperation with each other. In addition, the invention relates to an electronic device including the power supply system.
2. Description of the Related Art
Hitherto, there have been many proposals for power supply systems of the type wherein a power generator, such as a fuel cell or a solar cell, for performing power generation, and a power storage device, such as a secondary cell or a capacitor, for performing charging/discharging operation are combined. Example power supply systems of this type are shown in FIGS. 7A and 7B. For example, in a power supply system shown in FIG. 7A, a load 903 and a power generator 901 are interconnected, and a power storage device 902 is parallel connected to the power generator 901. In addition, in a power supply system shown in FIG. 7B, a constant voltage converter 904 is connected between the power generator 901 and power storage device 902 of FIG. 7A.
However, according to any of these power supply systems, in the event of charging of the power storage device 902, the charge voltage and the charge current are fluctuated by the amount of charge. As such, as shown in FIG. 8, a method has been proposed that uses a constant voltage and constant current converter 905 as a method of charging the power storage device 902.
In systems using a solar cell, a control scheme for operation with an operating point set to a maximum output point from the current and voltage characteristics thereof (MPPT (maximum power point tracker) scheme) is generally employed, whereby a power storage device 902 of the above-described type can be charged with high efficiency. It is known that in a fuel cell, the output current is fluctuated significantly by an external factor, such as the temperature. In many cases where current fluctuations occur in a load 903, the characteristics thereof are influenced significantly, to the extent of hindering a stable operation. As such, it is preferable to capture an output meeting output characteristics that is determined by the environmental conditions thereof.
However, in the case shown in FIG. 8, an output current Icc from the constant voltage and constant current converter 905 takes a value representing the sum of a charge current Ichg and a current Ild demanded by the load 903, that is, Icc=Ichg+Ild. In this case, since the output current Icc from the constant voltage and constant current converter 905 is constant, when the current Ild demanded by the load 903 fluctuates, the charge current Ichg also fluctuates, and care should be taken so that the power storage device 902 is not overcharged.
In addition, the following techniques have been reported. One of them is a fuel cell system (see Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2002-231287), for example). In this case, a fuel cell is directly connected to a load, and the power storage means circuit containing a power storage means is connected parallel with the fuel cell, wherein the power storage means circuit controls a charge current to the fuel cell. Another technique is a hybrid power system (see Patent Document 2 (Japanese Unexamined Patent Application Publication No. 60-109728), for example). In this case, in a hybrid system of a fuel cell containing a switch and a charger cell containing a switch, the switches are controlled to thereby control charging and discharging of the charger cell. Another technique is a DC hybrid supply system (see Patent Document 3 (Japanese Unexamined Patent Application Publication No. 07-320752), for example). In this case, the DC hybrid supply system has a power supply unit connected to a load through a DC/DC converter means, a power storage device parallel connected to the power supply unit, and a DC/DC converter means, wherein a control means controls power supply to the load and charging and discharging of the power storage device.
However, in the fuel cell system according to Patent Document 1, the power storage means is charged by the fuel cell, the regeneration current, and the like. In addition, the control is performed through switching when the amount of stored power is insufficient. As such, the value of charge current to the power storage means and the value of charge voltage to thereto are not controlled. Consequently, the power storage means cannot be charged with a stable current value and a stable voltage value.
In the power system according to Patent Document 2, the control through the on or off operation of the switch is performed in response to the charge status of the charger cell. However, the control through the on or off operation of the switch is not beyond a selection whether or not the charger cell is to be charged, so that the charging voltage, the charging current, and the like to the charger cell cannot be controlled. Further, in the supply system according to Patent Document 3, while only an allowable maximum voltage value and an allowable minimum voltage value of the power storage device are provided, the charge voltage, the charge current, and the like in the charging event are not controlled. Consequently, in the power system according to Patent Document 2 and in the supply system according to Patent Document 3, the charge voltage, the charge current, and the like are fluctuated undesirably by the amount of charge of the charger cell, consequently disabling stable charging.